1. Field of the Invention
The invention relates to a layout structure of a MOSFET and a layout method thereof. More particularly, the invention relates to a layout structure of a MOSFET in which a plurality of MOSFET cells substantially has the same amount of the current passing there through and a layout method thereof.
2. Description of Related Art
Currently, for the main devices of the computer, such as the CPU and the memory, their operation voltages have gradually become lower. Under the situation in which the required power consumption can not become lower in the same ratio, the required current of these devices for the operation gradually becomes larger. Accordingly, for a MOSFET severing as a power switch in the application, the maximum with standing current of the MOSFET also becomes higher along with the required current. However, with the development of the high integration in the integrated circuit, the width of the metal conducting wire connecting the source and the drain of the MOSFET would be limited, so that the current density of the metal conducting wire is increased, and the metal conducting wire has a serious voltage drop. The serious voltage drop causes the conducting current conducted in the MOSFET cells of the MOSFET to be non-uniform. The increased current density of the metal conducting wire causes the electron migration to become obvious. Especially, if the conducting current is non-uniform, the MOSFET cell that conducts the larger conducting current is easily destroyed due to the electron migration. Accordingly, the conventional power MOSFET has either the character of the large withstanding current or the character of the high integration.
FIG. 1 is a schematic view of a conventional layout structure of the MOSFET. Referring to FIG. 1, in the MOSFET structure, the gate conducting wire has short strip-shape gate patterns 31 and long strip-shape gate patterns 32, which wiggles and connects the gates of each MOSFET cell in the MOSFET structure. The source regions 14 and the drain regions 24 of each MOSFET cell in the MOSFET structure are alternately located between the long strip-shape gate patterns 32. The drain metal conducting wires 21 have a plurality of finger-like patterns 22, and the source metal conducting wires 11 have a plurality of finger-like patterns 12. The finger-like patterns 22 and 12 are alternately disposed between each other. Each of the finger-like patterns 22 is located on a drain region 24, and is electrically connected to the drains of the MOSFET cells through the drain contact holes 23. Similarly, each of the finger-like patterns 12 is located on a source region 14, and is electrically connected to the sources of the MOSFET cells through the source contact holes 13. Moreover, each of the finger-like patterns 22 has a plurality of drain contact holes 23 to be connected to the drain region 24. Each of the finger-like patterns 12 has a plurality of source contact holes 13 to be connected to the source region 14.
In the conventional layout structure of the MOSFET, the over long source connecting wires and the over long drain connecting wires cause the serious voltage drop. FIG. 2 is a schematic view of the gate, the drain, and the source voltages of the MOSFET cells in the MOSFET structure shown in FIG. 1 changing along with the distance current flowing through the connecting wires. Herein, the line Svol represents the source voltage, the line Dvol represents the drain voltage, and the line Gvol represents the gate voltage. For clearly describing the relationship between FIG. 1 and FIG. 2, a current direction Z is defined in FIG. 1 herein. Accordingly, the current I flows into the MOSFET from the drain metal conducting wires 21, and flows out thereof from the source metal conducting wires 11. Due to the current, the source voltage Svol and the drain voltage Dvol are decreased along with the distance, and the differences between the source voltage Svol and the ideal source voltage Svol′ and between the drain voltage Dvol and the ideal drain voltage Dvol′ gradually become larger. Moreover, there is no current flowing into the gate of the MOSFET, so that the gate voltage Gvol does not change along with distance. In this case, the gate-source voltages Vgs of each MOSFET cell are different, and it causes the current conducted in each MOSFET cell to be different. Especially, for the requirement of the large current, the number of the MOSFET cells is increased, and the lengths of the metal conducting wires are also increased, so that the current conducted in each MOSFET cell is much different.
Accordingly, the design for the MOSFET structure has the intrinsic defect and limitation, and it can not satisfy the requirement of the large current in recent advancement.